[CI] pin nixos/nix docker image, use experimental features flakes

2 jobs for dev-spago-next-with-flake in 8 minutes and 9 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Compile
passed #12906
compile

00:05:47

 
  Test
failed #12907
test

00:02:21

 
Name Stage Failure
failed
test Test
building '/nix/store/7sphagv6hn8m4yr07lr8swnzrgwslnyw-spago-0.93.29-npm-deps.drv'...
copying path '/nix/store/8mjb3ziimfi3rki71q4s0916xkm4cm5p-gcc-wrapper-13.2.0' from 'https://cache.nixos.org'...
copying path '/nix/store/kkjn1zwdvmywwbbijhj5ss4ca4nzggxb-node-gyp-10.0.1' from 'https://cache.nixos.org'...
copying path '/nix/store/558iw5j1bk7z6wrg8cp96q2rx03jqj1v-stdenv-linux' from 'https://cache.nixos.org'...
building '/nix/store/6sq0mi4qkd8whdqqpghwqx5kv3j48hj5-spago-0.93.29.drv'...
building '/nix/store/syqbj5jr03nc6axhd9mhs61vckyb099m-fix-bun.drv'...
/nix/store/bdiz4v04cvpxlg04jw9kq7i9qbmwxfh0-fix-bun/bin/fix-bun: line 11: cd: ./node_modules/better-sqlite3: No such file or directory
Cleaning up file based variables
ERROR: Job failed: exit code 1